1. Field of the Invention
The present invention relates to a driver circuit for use in a display device and, more particularly, to a driver circuit adapted for use in an active matrix liquid crystal display.
2. Description of the Prior Art
Heretofore, a driver circuit for use in a display device such as an active matrix liquid crystal display has adopted line-sequential scanning making use of shift registers.
The whole prior art liquid crystal display is schematically shown in FIG. 1. A signal line driver circuit 101 and a scanning line driver circuit 102 are formed on the same glass substrate. Also, a liquid crystal pixel portion 103 is created in the center of the display device.
The driver circuits 101 and 102 are connected with the liquid crystal pixel portion 103 by signal lines X1, X2, . . . extending in the direction of columns and by signal lines Y1, Y2, . . . extending in the direction of rows. Thin-film transistors (TFTs) acting as switching devices are formed at the intersections of the signal lines and the scanning lines. That is, the TFTs are arranged in rows and columns.
The source electrodes of the TFTs are connected with the signal lines. The gate electrodes are connected with the scanning lines. The drain electrodes are connected with the pixel electrodes, which are located on the opposite side of a liquid crystal material from a counter electrode (not shown).
The signal lines are sequentially scanned by the signal line driver circuit 101. In synchronism with this scanning, signals are supplied to the liquid crystal pixel portion 103 via the scanning lines from the scanning line driver circuit 102. In this way, signals necessary to provide a display of images are applied to the liquid crystal pixel portion 103.
The line-sequential scanning is now described in detail. One input signal is transmitted with a delay. The signal lines in the scanning line driver circuit are sequentially scanned. Every transistor on one scanning line is once driven into conduction. Signals are supplied to signal storage capacitors via the signal lines from the signal line driver circuit. The supplied signals keep the liquid crystal material activated until scanning for the next frame is started.
At this time, if a constant voltage is kept applied to the liquid crystal material, then it will be deteriorated. In order to prevent this, the polarity of the display signal applied to the liquid crystal material is reversed every frame. In particular, the voltage applied to the source of each TFT forming a pixel is changed from a reference voltage of +10 V to +5 V and from the reference voltage to −5 V, and so on.
In the line-sequential scanning method described above, n stages of shift register circuits connected in series are employed to delay signals. The shift register circuits are made up of flip-flops. In the case of the signal line driver circuit, the number of stages n of the connected shift register circuits is the number of pixels in the horizontal direction. In the case of the scanning line driver circuit, the number of stages n is the number of pixels in the vertical direction.
The output signal from the shift register circuits connected in series is sent to the next stage of shift register circuit, delayed, and transmitted. Signal conversion circuits and amplification circuits such as analog memories and inverters are connected in series with the outputs of the shift register circuits.
FIG. 2 is a block diagram of an analog line-sequential driver circuit. This circuit includes a signal line driver circuit 200 and a scanning line driver circuit 201. The signal line driver circuit 200 consists of a shift register circuit composed of flip-flops connected in series. Power voltages Vdd (202) and Vss (203) are applied to the signal line driver circuit 200. Also, clock pulses CP (204) are applied to the signal line driver circuit 200. An applied start pulse SP (205) is passed through the flip-flops with delays in the direction of scanning (e.g., to the right), the flip-flops being connected in series inside the signal line driver circuit 200.
The shift registers deliver output signals Q0, Q1, . . . , Qn, respectively. Using these output signals as timing signals, a video signal 206 indicating data about gray levels is sampled by a sampling circuit using an analog switch 207.
The sampled data about the gray levels is once stored in an analog memory 208 before applied to the pixel portion. The stored data is scanned at the timing determined by latch pulses 209 supplied from the outside. The signal is subjected to an impedance transformation in an analog buffer 210. Then, the signal is sent to a pixel TFT 212 through a signal line 211. In each stage of the signal line driver circuit 200, such a signal path is followed. As a result, an image is scanned along the successive lines sequentially.
In recent years, digital memories using latches have been increasingly employed instead of analog memories. That is, data signal is not stored in analog memories but applied to latches, where the image data is retained as a binary-coded digital signal.
By digitizing signals in this way, decreases of the life of gray-level display data as encountered in the analog configuration are avoided. Hence, stable gray-level signals can be obtained.
Furthermore, lower voltage and lower electric power consumption can be accomplished by utilizing the digital scheme. This, in turn, leads to lower costs. In addition, the operation speed can be made higher.
With the prior art display device driver circuit using shift register circuits, if any one of the shift register circuits connected in series is defective, then no signal is transmitted to the following stages of shift register circuits. This causes a decrease in the production yield of the whole display device.
Every signal necessary to provide a display is carried by one video signal and so a high voltage is necessitated. As a result, the electric power consumed is increased.
The video signal is passed via the sampling circuit to the analog memory (capacitor) and once stored there. Electric charge leaks from this analog memory. Therefore, it may not be possible to store a required amount of electric charge. This shortens the life of the display data signal. In consequence, the image quality is deteriorated.
Especially, where the driver circuits are made up of TFTs formed on a glass substrate or the like, the driver circuits occupy a broader area than driver circuits formed on a single-crystal substrate. Therefore, faults are more likely to occur. For this reason, a driver circuit and a liquid crystal display portion are integrally formed on a glass substrate. In the case of an active matrix liquid crystal display incorporating a peripheral circuit, faults tend to occur with the TFTs forming shift registers, thus deteriorating the production yield of the finished display device. As a result, the cost is increased.
In a line-sequential analog driver circuit, every necessary gray-level data is carried by only one video signal. Therefore, a high voltage is needed. This shortens the lifetime of the circuit made up of TFTs. The electric power consumed is inevitably increased.
Where an analog memory is used, there is the possibility that the life of the gray-level display data is shortened due to leakage of electric charge from capacitors. Therefore, it is difficult to accomplish high image quality.